1. Field of the Invention
The present invention relates to an optical pickup device, and particularly to an optical head.
2. Description of Related Art
A variety of optical discs such as those called LD (Laser Disc), CD (Compact Disc), and DVD (Digital Video Disc) are known as optical information recording media. These optical discs are separately standardized in accordance with different specifications for requirements such as the thickness of a substrate. For instance, in a multi-layer structured optical disc included in the DVD specifications, each recording layer has a different effective thickness of the optical disc. In addition, the different optical discs also require different optimal numerical apertures (NA) for optical lenses for reading information therefrom.
While there is a need for a compatible player which is capable of reading recording information from either of CD and DVD, an optical system for reading CD differs from an optical system for reading DVD in the following aspects:
(1) Difference in Numerical Aperture NA: 0.6 for DVD versus 0.45 for CD; PA1 (2) Difference in Thickness of Substrate from Optical Disc Surface to Recording (Reflecting) Surface: 0.6 mm for DVD versus 1.2 mm for CD; and PA1 (3) Difference in Wavelength of Optimal Reading Light: 650 nm for DVD versus 780 nm for CD.
It is therefore necessary to absorb these differences in specifications for realizing an optical pickup for a CD/DVD compatible player.
To realize such a compatible player, it is often thought to be convenient to employ an objective lens having two focuses in an optical pickup. Specifically, an objective lens having a single focus is not suitable because an optimal numerical aperture for one optical disc is not appropriate for the other optical disc, and employment of an inappropriate disc will result in the generation of aberration such as spherical aberration or the like.
An example of realizing a two-focus pickup has already been known (Japanese Patent Application Kokai No. 7-98431) in which a hologram lens is employed. This two-focus pickup has a composite objective lens formed of an objective lens 1 and a hologram lens 2, as illustrated in FIGS. 1 and 2. The hologram lens 2 is formed with a plurality of concentric diffraction grooves 3 extending over a region corresponding to a numerical aperture for CD. The diffraction grooves 3 deflects a light beam from a light source to split the light beam into zero-order diffracted light and first-order diffracted light to provide them with different incident angles to the objective lens 1, thereby focusing the light beam on a recording surface of each optical disc as required. In this event, the optical beam as it is transmits a region of hologram lens 2 without the diffraction grooves 3 and is converged by the objective lens 1 together with the zero-order diffracted light, so that the transmitted light as well as the zero-order diffracted light and the first-order diffracted light cause a change in numerical aperture. The first-order diffracted light diffracted by the diffraction grooves 3 and having a smaller numerical aperture is utilized for reading a CD 4 as illustrated in FIG. 1, while the transmitted light and the zero-order diffracted light having a larger numerical aperture is utilized for reading a DVD 5 as illustrated in FIG. 2.
Basically, the foregoing structure forms the reading spots with a single light source, and generally uses a light source which emits a reading light having a wavelength of 650 nm appropriate to the DVD. However, appropriate light for reading recordable and reproducible recording media called CD-R (CD Recordable, or R-CD: Recordable CD) belonging to a CD family is 780 nm in wavelength. In other words, the light at wavelength of 650 nm is not appropriate for reading information recorded on a CD-R. Therefore, it can be said that, to realize a compatible player capable of favorably recording and reproducing information on and from a disc such as the CD-R, it is safer to provide a modified structure for an optical pickup or a head having a plurality of light sources for emitting light at different wavelengths appropriate to the respective discs, instead of the structure employing a single light source for emitting light at a single wavelength. However, an easily designed optical system having a plurality of light sources would result in an optical pickup or a head which tends to have a more complicated structure and a larger size as a whole.